This invention relates to a shut-off system for the avoidance of an overspeed condition in the event of a shaft failure, in particular for the interruption of the fuel supply of an aircraft engine in the event of a failure of the low-pressure turbine shaft, with the energy-consuming end of the respective shaft being connected to a coaxial reference shaft.
In particular on aircraft engines, but also on turbomachines in general, a shaft failure, especially a failure of the low-pressure turbine shaft, constitutes a considerable hazard to persons and property. In the event of a failure of a turbine shaft, the energy-generating end of the shaft, i.e. the part of the shaft which is connected to the turbine rotor, will run up in an uncontrolled manner since it is now disconnected from the energy-consuming end of the shaft. Consequently, the engine and/or the aircraft may be damaged or destroyed.
Various devices for the mechanical and/or electronic detection of a shaft failure and for the subsequent interruption of the energy supply (fuel supply) to avoid an overspeed condition and its negative consequences are known. Here, it is crucial that a shaft failure is detected as early as possible and the engine is shut off instantly by interruption of the fuel supply.
In an electronic device for the detection or avoidance of an overspeed condition known from Patent Specification U.S. Pat. No. 4,712,372, two inductive sensors are arranged on the toothed turbine shaft, i.e. on the energy-consuming end and on the energy-generating end, which produce a speed-proportional signal corresponding to the number of pulses counted. If a speed difference resulting from an increase of the speed of that part of the shaft which is connected to the turbine rotor, and thus a shaft failure, is detected, a solenoid fuel valve will be actuated and the fuel supply interrupted, avoiding further acceleration of the turbine rotor. However, the electronic shut-off systems are critical in that their response times are relatively long. For safety reasons, relatively long shut-off times require a higher material input in the area of the turbine which, in turn, leads to an increase in weight.
Furthermore, mechanical devices are described in which a reference shaft is coaxially associated to the turbine shaft and connected to the forward, energy-consuming end of the turbine shaft. In the event of a shaft failure, the resultant rotation of the turbine shaft relative to the reference shaft is used to mechanically actuate the fuel shut-off valve. In a known mechanical device of this type for the control of overspeed conditions in the event of a failure of the low-pressure turbine shaft connecting the low-pressure turbine and the fan of an aircraft engine, recesses are provided on the rear ends of both shafts. In the event of a shaft failure, the low-pressure turbine shaft will rotate relative to the reference shaft and the—initially offset—recesses in both shafts will come into coincidence, as a result of which a pre-loaded driver provided on the low-pressure turbine shaft will swing out radially and engage a wire loop provided at the end of a wire rope. The pull exerted on the wire rope is transmitted to a fuel shut-off valve to close it, thus limiting the overspeed condition by interrupting the fuel supply. The known mechanical devices using a reference shaft are disadvantageous in that their response depends on a comparatively large angle of relative rotation between the turbine shaft and the reference shaft. Also, the purely mechanical design of the shut-off system, and, in particular, the wire rope connection between the turbine shaft and the fuel shut-off valve, incurs high design effort and is susceptible to wear.